EPMA maps unveil the actual chemical variations and crystallization sequence of pyroxenes and plagioclase solidified from a basaltic liquid at variable cooling rates

Gennaro E; Radica F; Iezzi G; Vetere F; Nazzari M; Zellmer GF; Scarlato P

EPMA maps unveil the actual chemical variations and crystallization sequence of pyroxenes and plagioclase solidified from a basaltic liquid at variable cooling rates

dc.citation.issue	5 December 2023
dc.citation.volume	640
dc.contributor.author	Gennaro E
dc.contributor.author	Radica F
dc.contributor.author	Iezzi G
dc.contributor.author	Vetere F
dc.contributor.author	Nazzari M
dc.contributor.author	Zellmer GF
dc.contributor.author	Scarlato P
dc.contributor.editor	Romano C
dc.date.accessioned	2024-08-25T22:54:01Z
dc.date.available	2024-08-25T22:54:01Z
dc.date.issued	2023-12-05
dc.description.abstract	Crystal-chemical variations of pyroxene (px) and plagioclase (plg) have been analysed by X-ray electron microprobe (EPMA) mapping to quantify their actual chemical dispersions. These phases were experimentally crystallised from a basaltic liquid (B100, MORB from Iceland) at cooling rates of 1, 7, 60 and 180 ◦C/h from 1300 ◦C down to 800 ◦C. Experiments were run at ambient conditions applying defined temperature paths mirroring characteristic cooling rates from innermost to outermost portions of metre- to centimetre-thick lavas, dikes and bombs emplaced under submarine to subaerial conditions. As the cooling rate increases from 1 to 180 ◦C/h, the run-products become progressively enriched in pyroxene and depleted in plagioclase, while spinel is invariably low (few area%) and glass is significant only at 180 ◦C/h. An increase of cooling rate generally leads to enrichment of Al2O3 and depletion of MgO in px, while the opposite behaviour is observed for plg; these trends are mirrored by calculated cations (apfu: atom per formula unit) and components. Average variations as a function of cooling rate are similar to those already observed through classical analysis performed by single point EPMA. However, the actual chemical distributions of CaO versus MgO, Al2O3 and FeOtot oxides unveil the presence of a wider range in pyroxene chemistry. In particular, one px (px-1, CaO-rich, diopsidic type) is present at all the applied cooling rates; a very low CaO-px (px-2, pigeonite or orthopyroxene type) is detected at 1 ◦C/h; and, finally, once more population of px (px-3, CaO-poor diopsidic type) appears at 60 and 180 ◦C/h. By contrast, plg analyses yield invariably identical compositions. Textural variations as a function of cooling rate and geo thermometric estimations indicate that px-1 crystallised at high-T (or low ΔT), while plg mainly grew in the residual melt produced by the saturation of px. If only textures were evaluated, this order of segregation would like remain unrecognised since px at low cooling rates is smaller than plg. The abundance of phases, their crystal chemical features, and their order of segregation can be regarded through a theoretical framework of a time temperature-transformation (TTT) diagram. The most significant chemical variations are displayed by MgO and Al2O3 for both px and plg, which faithfully capture the evolution of cooling conditions. The chemical compositions of px-1 is close to the thermodynamic equilibrium only at 1 ◦C/h. As the cooling rates increase, the px chemistry indicates disequilibrium conditions. Finally, this study shows that as ΔT/Δt increases, the most abundant px (px-1) and plg are forced towards compositions that become progressively closer to those of the parental liquid.
dc.description.confidential	false
dc.identifier.citation	Gennaro E, Radica F, Iezzi G, Vetere F, Nazzari M, Zellmer GF, Scarlato P. (2023). EPMA maps unveil the actual chemical variations and crystallisation sequence of pyroxene and plagioclase solidified from a basaltic liquid at variable cooling rates. Chemical Geology. 640. 5 December 2023.
dc.identifier.doi	10.1016/j.chemgeo.2023.121752
dc.identifier.eissn	1872-6836
dc.identifier.elements-type	journal-article
dc.identifier.issn	0009-2541
dc.identifier.number	121752
dc.identifier.pii	S0009254123004527
dc.identifier.uri	https://mro.massey.ac.nz/handle/10179/71375
dc.language	English
dc.publisher	Elsevier B V
dc.publisher.uri	https://www.sciencedirect.com/science/article/pii/S0009254123004527
dc.relation.isPartOf	Chemical Geology
dc.rights	(c) 2023 The Author/s
dc.rights	CC BY 4.0
dc.rights.uri	https://creativecommons.org/licenses/by/4.0/
dc.subject	Basalt
dc.subject	Cooling rate
dc.subject	Kinetics
dc.subject	Plagioclase
dc.subject	Pyroxene
dc.title	EPMA maps unveil the actual chemical variations and crystallization sequence of pyroxenes and plagioclase solidified from a basaltic liquid at variable cooling rates
dc.type	Journal article
pubs.elements-id	484050
pubs.organisational-group	Other